Multiple component delivery pumps are used when two or more materials have to be combined to obtain a desired product. In many instances the process has to ensure that the ratio between the components is maintained accurately and continuously. This is true for example when components of plastic materials are delivered which cure by chemical reaction. In some industries like the Painting industry where multi component plastic materials ae applied by airless spray an additional requirement is that the delivery takes place under high pressure. This limits the delivery pumps to the piston type.
Piston pumps can be assembled to a multi component delivery apparatus by connecting them mechanically and operating them from one power source or by controlling a number of independent units via their propelling medium each unit consisting of a piston pump directly connected to a piston motor.
Several methods are known to operate independent piston pump/piston motor units simultaneously.
One such apparatus (DE-OS 2646 606) uses the propelling medium in all motors in sequence thus ensuring that the strokes begin and end at the same time. The stroke length can be adjusted to a required ratio by using piston motors of different diameters but same volume or by attaching so called bypass cylinders.
Another apparatus controls the motor piston movements by the volumetric ratio of the propelling medium. An electronic control unit in connection with sensors permanently checks the position of each piston compares the positions with each other and with a pre-selected ratio and in case of differences corrects the supply or exhaust of the propelling medium via motor operated valves or it reverses the piston strokes when the pre-selected dead centres are reached.
In these two versions the method of central control leads to acceptable results with respect to continuous ratio accuracy and stroke adjustments when hydraulic propelling medium are used. Because of the compressibility of air these versions are not suited for air motors.
An apparatus operated by compressed air (GB 12 45 097) controls all motors via an air selector actuated by one of the motors only. The method is described as "master-slave-system". With a restrictor the air rate admitted to the master motor is kept at a lower value than that admitted to the slave motors with the intention of allowing the slave motors to finish their strokes before the master motor does. The stroke length can be adjusted mechanically to a required ratio by a stroke stop.
This version of central control does not exclude that one or more of the slave pumps have to wait at their dead centre before being reversed or that they do not completely finish their previous stroke. It therefore does not guarantee that a preset ratio is continuously maintained especially when the viscosity of the product changes during the delivery process. The method of stroke length modification applied creates a free cylinder space which has to be filled with propelling medium before the piston is actuated. It therefore leads to delayed stroke commencement and to pulsation. Other methods of stroke length modification known (DE-OS 29 11 443 and DE-OS 2212997) have the same disadvantage in principle: they lead to pulsation either due to a dead stroke or due to a free cylinder space.